There may be no such thing as a free lunch, but there's plenty of free dinner on offer at one swanky Upper East Side restaurant this week. The catch? You have to be female, and you have to be wearing a school uniform.

Those were the rules laid down by the Park Avenue Winter restaurant to celebrate its Monday night appearance on the CW's high-school drama "Gossip Girl", and hordes of willing ladies have complied.

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"I called the restaurant because I thought it was probably a hoax," laughed Deborah Fenker, 29, a Manhattan model who saw the offer on a food blog and arrived with two friends, clad in a white shirt, black blazer and mini-kilt.

Confessing that the restaurant has been on her wish-list for months, she was thrilled to discover that the free three course meal, which usually sets diners back around $90 each, was no joke.

"I thought it would be an appetizer and an entrée and we'd have to pay for alcohol, but we didn't have to pay for anything," she exclaimed.

Of course, the promise of a free meal that includes a choice of filet mignon and or miso-glazed lobster has prompted a few people to bend the rules.

"We've had some guys coming in drag, and we've had to tell people that a T-shirt with their school's name on it doesn't count," noted Park Avenue owner Michael Stillman, who admits he's already "taken a little flack for being sexist" despite giving away hundreds of free meals.

"We just thought it sounded funny and unique," he shrugged, "And every girl I know seems to love it. We've had a diverse mix of people from all over the city. Even some of our older regulars have come in with their wives dressed as schoolgirls."

Now fully booked for the rest of the week, the chic dining room looks more like a crowded all-girls' school canteen by 9 p.m.

"I enjoy doing the schoolgirl thing once in a while – and guys are into it," giggled Carmen Flores, 24, a financial specialist and Blair Waldorf lookalike who dined with four friends.

"During the recession, there should be more "Gossip Girl" nights out," she declared.

Unfortunately, after Friday night, the schoolgirl dinners will be no more, although Stillman isn't adverse to future possibilities.

"I'm impressed with how much fun people are having," he noted, "So I guess it depends what TV shows we get on."